It’s with great pleasure that I release this freebies set containing 64 free Illustrator Brushes of pen marker strokes and scribbles. They’re great to give any design a more natural, hand-drawn look. All brushes are of the highest quality and you’re free to use them in both personal and commercial works. They are compatible with Illustrator versions from CS upwards. It’s worth mentioning that with brushes like these the best results can sometimes be attained by setting the stroke to 0.25px or less.
And here’s how you install them: Download and extract the .ai file from the archive. Move it to a folder of your choice (Illustrator stores all its default brushes in the directory Illustrator > Presets > Brushes). Open any illustrator document and go to the brushes palette. From the dropdown menu select: Open Brush Library > Other Library and choose the .ai file you just downloaded. That’s it. Enjoy!

@Matt & @Brian: You are correct when you say it is an empty artboard. This is no ordinary AI file, but a AI Brush file. If you load it, as explained in the article, you will see that the brushes are loaded in your brushes palette. Also, if you open the file in Illustrator as you would do with another file, you get an empty artboard, but check your brushes palette, you’ll see that the brushes are there.